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Focus

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A position where rays of light converge through a lens to create a sharp image. (This position is sometimes also called an "image point".)
"To focus" means adjusting the distance setting on a lens to create this image point, which defines the subject sharply.
With a photo camera, this is done by moving the lens physically towards or away from the film or sensor, or by moving the front of the lens towards or away from the rear part of the lens, which alters the focal length. (There are exceptions to this method: Contax, for instance, has a system where the autofocus operation actually moves the film and not the the lens.)
When something is "in focus", it's sharp.

Related Terms

Method of focusing the lens, using either a rotating mechanical action or a motorised method, where you enter a distance on the camera. Cameras with mechanical actions often have an aid to help you focus correctly. On some cameras you see the image split in half and as you adjust focus the two halves come together. When the images are lined up the focusing is achieved. Others have an LED indicator that blinks when out of focus and either goes out or stays on when focusing is correct.

With focus stacking (also simply called 'stacking') one combines multiple images in a software program, each image with a slightly different focus, in order to increase the depth of field or the sharpness of an image. It is mainly used for astronomy, macro or micro photography.

The action of adjusting the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method but using a motorised focusing ring.
Buying advice: Autofocus is essential if you want to use the camera effortlessly, but occasionally the system is fooled, so make sure the camera has a focus lock or infinity setting or, better still, full manual control. Also check what the closest range is. Some only go down to about one meter which is useless if you want to fill the frame with a small object such as a flower or piece of jewellery.

The ring used to adjust the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method, using a motorised focusing ring. Lenses with a ring that allows a good grip make it easier to adjust and focus manually.

Bokeh is the English form of the Japanese word boke, which means blur. It refers to how out-of-focus parts of a photo look. Different lenses produce different-looking out-of-focus backgrounds, depending on things like the number and shape of their aperture blades. In general it could be said that the softer the out-of-focus elements look, the more pleasing the bokeh.

A special design of lens that is highly corrected to avoid chromatic
aberration by bringing the three blue, red and green wavelengths to one point of
focus rather than the usual blue and green of an achromatic lens. Because red is
also brought to the same point of focus, you don't need to refocus when taking
infrared pictures with this type of lens.

This is technically known as a catadioptric lens and has an unusual
construction of mirrors and lens elements. As well as glass elements there are
mirrors at the front and rear to fold the light as it enters the lens. Although this
results in a body that's a little wider than normal, advantages are: 1. The lens is usually only half the physical length a regular lens of the same focal length would be, and 2. It's much lighter. Disadvantages are: 1. There is no adjustable aperture, so the user is forced to take all of his photos at a permanent aperture setting, usually f8, but sometimes even f11, which means you need plenty of light for taking photos; 2. Practically all mirror lenses use manual focus; 3. Highlights that are out of focus are, in some situations, shown as doughnut shapes (although some actually like this characteristic and consider it an advantage, not a disadvantage).

One of two popular methods of camera focusing systems, the other is active. Passive has the advantage over active because it isnt fooled by glass in between the camera and the subject. Its also not dependent on subject distance. It works by measuring the subjects contrast and as such its main downfall is when the subject has no contrast. In such situations, such as snow scene or low light, the lens will focus in and out and may never reach sharp focus. Fortunately many cameras with this system have manual override.

With stacking (also called focus stacking) one combines multiple images in a software program, each image with a slightly different focus, in order to increase the depth of field or the sharpness of an image. It is mainly used for astronomy, macro or micro photography.

A parafocal lens, after having achieved focus at a telephoto focal length, offers the possibility of zooming back to a wide angle length, meanwhile maintaining focus on the subject. Non-parafocal lenses have to be re-focused after zooming back.

MPO, or Multiple File Format, is a camera image format, first used on Fujifilm's Real 3D W1 camera. A file contains two or more separate jpeg images. On the Fujifilm camera it's a stereo pair that gives the realistic 3D effect when combined at the viewing stage.

On some Ricoh cameras MPOs are created from the Multi-Target AF mode, where seven photos are taken in quick succession, each at slightly different focus points. The files can then be merged to create a focus stack, with incredible depth-of-field.

A mode to lock the focus to a predetermined point. Useful when the subject is moving fast because you can take the photo as it reaches a certain point or when other influences could affect the focusing such as shooting through glass or in crowded locations.

A hollow tube that fits between the lens and camera body to extend the lens-to-film/sensor distance and increases its close-focus capability. Although extension tubes can be bought separately, they often come in sets of three tubes with different lengths, which can be used indivually, or in any given combination.

Glass or plastic item that goes in front of the camera or enlarger lens to either absorb and remove selected wavelengths of light or add special effects such as soft focus, starbursts or multiple images. There are different ways of attaching filters, for instance screwing a round filter in front of the lens, or sliding a square or rectangular filter in a filter holder which is attached to the lens with an adapter ring.

A focusing mode on many cameras and lenses that allows close up pictures of subjects to be taken with a range of 20cm or less.
Digital compact cameras are particularly suited to macro work because they have such short focal lengths. Many models can focus as close as 1cm.

When you adjust the lens or film standard of a large format camera by tilting
it sideways and then follow by applying swing to the same standard you’ll find
that it will not be parallel in any axis to the other standard. To ensure sharp
focus you then need to make multiple adjustments. A camera with yaw free
movements allows the photographer to tilt and swing either standard without
having to re-adjust after.

A button or lever on the camera that either stops the lens aperture down manually or electronically. This enables you to view the image at the aperture that will be used to take the picture. The view will be darker, but you will be able to see exactly what will and won't be sharp or in focus. See "Using the depth of field button" in the Techniques section.

A low-cost lens that doesn't have a focusing ring. Found in very basic cameras. The lens is set to a distance of around three meters and relies on the depth of field to bring everything from about one and a half meters to infinity. The quality is always a compromise over a lens with adjustable focusing.